Due to a recent trend toward high integration and miniaturization of semiconductor devices, it is required to form a thin film with high precision in a silicon oxide film forming process, e.g., a process for forming an oxide film such as a gate oxide film of a MOS semiconductor, an oxide film on a trench inner surface of a shallow trench isolation (STI) or the like. As for a conventional technique for forming such silicon oxide film, a thermal oxidation has been performed. In that case, however, it is difficult to form a thin film. Therefore, there is recently noticed a plasma oxidation process capable of forming a thin and high-quality oxide film with good controllability (e.g., International Publication No. WO 2004/073073).
Meanwhile, the plasma oxidation process described in International Publication No. WO 2004/073073 has a drawback in that its processing speed is slow in a recently desired low-temperature film formation performed at a temperature lower than or equal to about 600° C. Thus, there has been suggested a method of increasing a speed of forming a silicon oxide film by using a processing gas containing H2 gas in addition to O2 gas and a rare gas such as Ar gas or the like. Even when a silicon oxide film has oxygen deficiency, H2 gas also serves to suppress dielectric breakdown by compensating the oxygen deficiency.
However, when hydrogen is introduced into the film, comparatively weak hydrogen bonds exist in the film. Such bonds are easily cut by current stress during use, so that dangling bonds are formed, thereby deteriorating characteristics of the silicon oxide.